1. Field of the Invention
The present invention relates to a catalytic system for cellulose acylation and to processes for producing said catalytic system and its application for cellulose acylation and synthesis of acetone-soluble primary cellulose acetates.
The aforesaid catalytic system incorporates as the base an adduct of sulfuric acid and N,N-dimethylacetamide. There is no direct reference to the presence or formation of this adduct in the reaction medium for producing cellulose esters. At the same time, a number of patents describe nonindustrial processes for synthesis of cellulose esters, wherein this adduct may be formed and may function as a specific catalyst. Such processes are recommended for carrying into effect an accelerated process of cellulose acetylation at high temperatures, with the maintaining of a high degree of polymerization of the product obtained.
Cellulose esters produced with the use of this catalytic system are used in traditional fields, e.g., for producing artificial fibers, cigarette stick, cine and photographic materials, commercial and household-use plastics, lacquers and varnishes, surface-coating materials, and for other applications.
2. The Prior Art
U.S. Pat. No. 2,705,710 discloses a cellulose acetylation process which, (since no direct reference is given in this patent) incorporates formation of an adduct of sulfuric acid with N,N-dialkylamide (DAA) selected from the group consisting of N,N-dimethylformamide (DMFA), N,N-diethylformamide (DEFA), N,N-dimethylacetamide (DMAA), and N,N-diethylacetamide (DEAA). This adduct is dissolved in a large quantity of the respective amide. The inventors have to use a large proportion of amide (450 moles per mole of sulfuric acid) which enables one to suppress an excessively high-temperature dissociation of the adduct into free sulfuric acid and amide, thereby retaining an increased degree of polymerization of the cellulose triacetate being synthesized. This, however, results in too high an acetylation module and therefore complicates the process of regeneration of a mixture of a large number of solvents (acetic acid and N,N-dialkylamide).
Fraizy, U.S. Pat. No. 2,536,634 discloses a method for cellulose acetylation wherein cellulose is first treated with acetic acid, acetic anhydride and sulfuric acid. It is then treated with an acetylating mixture comprising acetic anhydride and sulfuric acid. 30.0 minutes later compounds of a basic character and used for partial or complete neutralization of sulfuric acid are added to the reaction medium. In particular, acetic acid is added, and this acid comprises acetamide in terms of 1.0 mole of acetamide per 1.0-2.0 moles of sulfuric acid.
The Fraizy method has the following disadvantages which do not permit to:
(a) implement commercial procedures with very low module, about 3.0 since the module used in the Fraizy method which is considerably higher, approximately 8.0-10.0 can not be reduced due to relatively low temperatures at the acetylation stage, at which temperatures only the Fraizy method can be implemented;
(b) produce cellulose acetates having a higher degree of polymerization since in this method at the beginning of cellulose treatment with the acetylating mixture (within the first 30 minutes), when the reaction proceeds in the presence of the total amount of sulfuric acid, the degree of cellulose polymerization decreases to a considerable extent; and
(c) control the degree of substitution of cellulose acetates during the acetylation process and to obtain acetone-soluble primary cellulose acetates, since this prior art method can not use large enough quantities of sulfuric acid required for achieving the specific goals.
U.S. Pat. No. 2,976,277 describes a process for producing primary acetone-soluble cellulose acetates, using a titanium compound as the catalyst, wherein a xe2x80x9chomogeneousxe2x80x9d acetylation mechanism takes place. A disadvantage inherent in this process resides in that acetylation is conducted in the medium of acetic anhydride with a high module of acetylation equal to 13, the amount of acetic anhydride used being more than 7 times that required according to theory. In addition, the process is conducted at rather high temperatures equal to 136xc2x0 C.-137xc2x0 C.
One prior art process for producing primary low-substituted cellulose esters is described in Japanese Patent 57-35451. This process features homogeneous cellulose acylation conditions (in a cellulose solution in N,N-dimethylacetamide with lithium chloride), and the resultant formation of low-substituted cellulose esters, including cellulose acetate soluble in an acetone-water mixture, the acetylation module being about 40. This prior art process, however, fails to produce primary acetone-soluble cellulose acetates and features a complicated process of regeneration of a mixture of great amounts of solvents, i.e., N,N-dimethylacetamide and acetic acid.
One more prior art process for producing acetone-soluble primary cellulose acetates is known (Chemical Abstracts, V. 52, No. 14, Jul. 25, 1958, p. 14169a), wherein the process is conducted in a medium containing acetic acid and acetic anhydride, with a much lower acetylation module than specified above, that is, 8 at a temperature of below 33xc2x0 C. According to this prior art process, cellulose acetylation is carried out in the presence of a large amount of free sulfuric acid, i.e., 20%-45% of the weight of cellulose. A disadvantage of the process is a relatively low degree of polymerization of the resultant product, that is, 160, as well as comparatively high acetylation module equal to 8.
Soviet Union Patent No. 1,237,671 discloses a process for producing primary cellulose acetates in a medium of N,N-dimethylformamide and used as the catalyst is a mixture called a xe2x80x9ccomplexxe2x80x9d by the inventors, comprising 600-800 wt.pt. of N,N-dimethylformamide and 20-40 wt.pt. of sulfuric acid. Primary cellulose acetates are produced as follows: the parent cellulose is activated with this xe2x80x9ccomplexxe2x80x9d at 60xc2x0 C.-80xc2x0 C. for one hour, whereupon acetylation is carried out in the presence of this xe2x80x9ccomplexxe2x80x9d. The resultant product is subjected to desulfation, the catalyst is neutralized, and the end product is extracted by a conventional technique. The end product is acetone-soluble and has a specific viscosity of 0.36-0.68. Disadvantages of the process reside in a high acetylation module equal to 10 and a complicated process of regeneration of a mixture of a great amount of solvent, i.e., N,N-dimethylformamide and acetic acid. In addition, this prior art patent contains an inexact information about the composition of the xe2x80x9ccomplexxe2x80x9d used, which is apparently a solution of the adduct of N,N-dimethylformamide and sulfuric acid in N,N-dimethylformamide.
It is an object of the present invention to provide a catalytic system which is capable of synthesizing cellulose esters with a minimized acylation module equal to 3-7, preferably 3.0-5.0, in a medium of a lower fatty acid-anhydride of a lower fatty acid.
The foregoing object is accomplished due to the provision of a catalytic system for cellulose acylation, said system comprised in an acylating mixture composition and comprising an adduct of sulfuric acid with N,N-dimethylacetamide as well as free sulfuric acid or free N,N-dimethylacetamide with the following mole ratio of the components:
The principal component of the catalytic system of the invention is an adduct, i.e., an equimolar complex compound of sulfuric acid with N,N-dimethylacetamide.
It has now been discovered that this complex in a catalytic system with free sulfuric acid incorporates an N-cation, and the use of free N,N-dimethylacetamide in the catalytic system of the invention results in formation of a complex containing both an N-cation and O-cation at a time, whereas the N-cation alone is present in an equimolecular system.
It has also been discovered that an adduct with an N-cation is less stable than an adduct containing both an N-cation and O-cation. This fact is evidenced by a difference, of more than two orders of magnitude, between the values of instability constants of the complex with an N-cation and with an O-cation, calculated by the calorimetric titration technique (at a temperature of 25xc2x0 C. and concentration of the titratable component in acetic acid equal to 0.05 M/l).
The complex composition has been studied by us with the use of IR-spectroscopy in the region of carbonyl group valence vibrations. In this case, the spectrum of a complex with an N-cation is featured by an absorption band of 1700 cmxe2x88x921, and the spectrum of a complex containing an O-cation has an absorption band of 1600 cmxe2x88x921.
The other important component of the proposed catalytic system is free sulfuric acid or free N,N-dimethylacetamide. The catalytic system containing free sulfuric acid has a greater number of protons which are formed due to both free sulfuric acid and lower stability of the complex with an N-cation. This catalytic system is more active in the cellulose acylation processes than a system containing free N, N-dimethylacetamide.
The limit values of the ratio between the components of the proposed catalytic system which determine its practical application for cellulose acylation processes, have been selected experimentally.
Any extension of the thus-selected limits of the values of the ratio between components of the herein-proposed catalytic system results in a reduced degree of polymerization of the finished product and/or an increased duration of the cellulose acylation process.
Similarly, the use of a catalytic system containing an adduct alone of sulfuric acid with N,N-dimethylacetamide (without their excess amount) fails to yield any gain both in a higher degree of polymerization of the finished product and in a reduced time of the cellulose acylation process.
The herein-proposed catalytic system of the invention may also incorporate an adduct of perchloric acid with N,N-dimethylacetamide in an amount of 50 wt. % of the amount of an adduct of sulfuric acid with N,N-dimethylacetamide. This catalytic system is recommended for synthesis of mixed cellulose esters (i.e., cellulose acetoproprionates and acetobutyrates), because this catalytic system enables one to complete the cellulose acylation reaction using butyric anhydride or propionic anhydride which are less chemically reactive compared with acetic anhydride. The aforesaid ratio between the components of a catalytic system comprising an adduct of perchloric acid with N,N-dimethylacetamide, provides for a synthesis of mixed cellulose esters featuring high substitution with respect to butyric acid or propionic acid and a low acetylation module. Any change in this ratio between the adducts of sulfuric acid and perchloric acid adversely affects the quality of the finished product.
Depending on a production process applied the herein-proposed catalytic system may additionally comprises a lower fatty acid having from 2 to 4 carbon atoms (that is, acetic acid, propionic acid and/or butyric acid) in an amount equal to 2 moles per mole of water contained in the parent components, or in an amount of not more than 14.0 moles per 1.0 mole of an adduct of sulfuric acid with N, N-dimethylacetamide.
The process for producing the herein-proposed catalytic system consists in that sulfuric acid is made to react with N,N-dialkylacetamide in an anhydrous medium, wherein, according to the invention, the N,N-dialkylacetamide used is N,N-dimethylacetamide, and the process is conducted in the absence of cellulose at a temperature of 0xc2x0 C.-25xc2x0 C. and with the following mole ratio of the parent components:
When preparing the catalytic system, 0.67-1.40 moles of DMAA are taken per mole of sulfuric acid. The present catalytic system is prepared under anhydrous conditions.
Such anhydrous conditions can be established either by using water-free parent components or by adding an aliphatic-acid anhydride in an amount of at least one mole of the anhydride per mole of water for this water removal by anhydride hydration.
According to the invention, the catalytic system is prepared from N,N-dimethylacetamide and sulfuric acid independently, outside the reaction medium containing cellulose. This makes it possible to prevent production of a substandard cellulose ester under low-module conditions, since there are avoided non-uniform processes of cellulose destruction and acylation which are caused by two concurrently proceeding reactions, i.e., anhydride hydration and adduct formation. Heat released during adduct formation and anhydride hydration, when preparing the proposed catalytic system, is withdrawn by cooling the system in such a way as to maintain its temperature between 0xc2x0 and 25xc2x0 C., thereby reliably controlling the process for producing a quality end product.
Sulfuric acid, which is one of the key parent components of the catalytic system of the invention, is selected on account of its being a strong mineral acid which, when in the composition of the present catalytic system, forms sulfoesters together with cellulose, thus rendering cellulose acetylation in a reaction running the xe2x80x9chomogeneousxe2x80x9d mechanism and ultimately contributing to realization of a low-module process.
The other important parent component of the catalytic system of the present invention is N,N-dimethylacetamide which occupies an extraordinary position in the series of N,N-dialkylamides of a lower fatty acids having 1 to 2 carbon atoms as being a specific solvent. As sulfuric acid is added to cellulose during its acylation, N,N-dimethylacetamide passes from the adduct to a free state, increases its concentration in the acylation bath, thereby adding to the dissolving capacity of, the liquid phase with respect to the cellulose acetosulfate formed. The aforesaid specific features of N,N-dimethylacetamide provide a unique possibility of carrying out a low-module cellulose acylation process at relatively low temperatures for an adequately short period of time.
One more component of the catalytic system of the invention is perchloric acid which is used as the strongest heretofore-known cellulose acylation catalyst. Perchloric acid is used in an amount of 50.0 wt. % of the amount of sulfuric acid. The catalytic system, wherein apart from sulfuric acid used is made of perchloric acid, is prepared in presence of a lower fatty anhydride.
The herein-proposed catalytic system based on the adduct of sulfuric acid, perchloric acid, and N,N-dimethylacetamide enables one to carry out low-module cellulose acylation processing producing acetopropionates of cellulose and acetobutyrates of cellulose.
The herein-proposed process for cellulose acetylation at low module comprises cellulose treatment (activation) with acetic acid, preparation of a catalytic system and an acetylating mixture, acetylation of activated cellulose with this acetylating mixture comprising at least a catalytic system and acetic anhydride. According to the invention, preparation of the catalytic system is conducted in the absence of cellulose; an adduct of sulfuric acid with N,N-dimethylacetamide and free sulfuric acid with mole ratio of 1.0:max 0.49 are used as a catalytic system. This system is added to the acetylating mixture in an amount of about 1.5 wt. % of cellulose weight (in terms of sulfuric acid), and the acetylation of activated cellulose is conducted with module about 3.0.
It is noteworthy that when the catalytic system of the invention is used in an amount of about 1.5 wt. %, the resultant cellulose triacetate is liable to gelatinize; that is why cellulose triacetate must be hydrolyzed before gelatination occurs.
With low module preferable use may be made in the proposed process of powdered cellulose, such as, e.g., wood or cotton cellulose. Along with retaining the principal requirements imposed upon the known cellulose grades for acetylation, it is also recommended that powdered cellulose have a bulk density in the range of 90-105 kg/cu.m and an adequately high degree of polymerization (not below 1200).
Use of powdered cellulose is especially efficient when the process is carried out in a reactor having high shear forces because this makes it possible to solve the problems of flows synchronization in the cellulose-liquid system which is indispensable in high-efficiency continuous processes for cellulose acetate production.
This becomes practicable due to a combined use of acetic acid as an activator, and the proposed catalytic system. To carry out the cellulose acylation process with a module of 3 it is expedient to use reactors featuring high shearing forces. With such a low acylation module the reaction medium is acetic anhydride, that is, the process occurs virtually without use of any organic solvents.
The present catalytic system should be used in an amount of 7-30% of the cellulose weight in terms of sulfuric acid and is applied with the following mole ratio of its components:
This catalytic system enables one to change, during the cellulose acetylation process, both the fixed acetic acid (i.e., degree of substitution) content of the resultant cellulose acetate and the degree of its polymerization, the latter being determined by the ratio between the adduct and the free N,N-dimethylacetamide in the system. The degree of substitution depends on the amount of the adduct used and is in an inverse relationship therewith. It is also associated with the amount of the transesterified cellulose sulfoesters which, in turn, depends on the temperature and the xe2x80x9csyrupxe2x80x9d residence time at the stage of acetylation. The cellulose acetylation process in the presence of the proposed catalytic system proceeds under the conditions approximating the homogeneous ones, which results in a more uniform distribution of the acetic-acid and sulfuric-acid residues along the chains of the macromolecules of the resulting cellulose acetosulfate, This is corroborated by the partially acetylated cellulose specimens obtained according to the invention in the presence of the proposed catalytic system, which contain much less amount of crystalline cellulose fraction unhydrolyzable with boiling 2.5N hydrochloric acid than the specimens of the same degree of substitution produced by the so-called homogeneous process.
In our opinion, a principal prerequisite for production of soluble primary cellulose acetates is a leading destruction of the cellulose crystalline structure (rendering it amorphous) accompanied by an increase in the number of hydroxyl groups accessible to reagents and by a greater proportion of the partially substituted units of macromolecules and their faster rendering into solution.
In the process of the invention for cellulose acetylation, in order to obtain cellulose acetates having a higher degree of polymerization and a controllable degree of substitution, the ratio between the liquid phase and the cellulose (module) is selected such that it equals 3.6-7.0:1.0, preferably 3.6-5.0:1.0.
When the cellulose acetylation process is conducted with a module of 3.6 to 7, the concentration of the reactants is increased, which cuts down the duration of the process, increases filterability and degree of polymerization of the finished product. Moreover, it is the xe2x80x9chomogeneousxe2x80x9d cellulose acetylation mechanism that is realized in this case, which provides a possibility of producing acetone-soluble primary cellulose acetate.
One of the advantages of the proposed process is a possibility of modernizing the now-existing industrial processes for producing cellulose acetates aimed at dispensing with, partially or completely, organic solvents, acetic acid, and methylenechloride in the reaction sphere. Thus, reduction of the acetylation module to 3.6-5.0 in the acetic-acid processes for producing cellulose acetates makes it possible to reduce the amount of acetic returned for regeneration by 1.5-2.5 times.
The selected range of the acetylation module values is an optimum one. Changing to either side of this optimum range will result in providing a substandard end product or will result in a considerable deterioration of the economic indices of the proposed cellulose acetylation process.
The other field for application of the proposed catalytic system is for producing acetone-soluble primary cellulose acetates having a higher degree of polymerization. This process comprises cellulose activation with acetic acid, preparation of the catalytic system and the acetylating mixture, acetylation of the activated cellulose with the acetylating mixture comprising at least the catalytic system and acetic anhydride. Treatment of the resultant cellulose acetate with water is followed by isolation of the end product; wherein, according to the invention, preparation of the catalytic system is conducted in the absence of cellulose. An adduct of sulfuric acid with N,N-dimethylacetamide and free N,N-dimethylacetamide with a mole ratio of 1.0:0.2-0.4 are used as a catalytic system. This system is added to the acetylating mixture in the amount of 20.0-30.0 wt. % of cellulose weight (in terms of sulfuric acid); acetylating of activated cellulose with said acetylating mixture at a temperature of 10xc2x0 C.-90xc2x0 C. and module equal to 4.0-7.0, preferably 4.0-5.0. Further treatment of the resultant cellulose acetate is performed with water, acetic acid and magnesium acetate or sodium acetate in two stages:
at the first stage, treatment is carried out at a temperature of 70xc2x0 C.-85xc2x0 C. for 1-2 hours with 20.0-50.0 wt. pt. of water, 25.0-50.0 wt. pt. of acetic acid and 10.0-40.0 wt. pt. of magnesium or sodium acetate per 100.0 wt. pt. of cellulose; and
at the second stage, treatment is performed at a temperature of 65xc2x0 C.-70xc2x0 C. for 1.0-2.0 hours with 45.0-100.0 wt. pt. of water, 45.0-105.0 wt. pt. acetic acid per 100.0 wt. pt. of cellulose.
The aforementioned two-stage treatment is aimed at desulfation, that is, splitting off the bound sulfuric acid making up part of the cellulose acetate and reestablishing free hydroxyl groups instead. At the first stage, the content of bound sulfuric acid is reduced to tenth fractions of a percent, after which the resultant cellulose acetate becomes soluble in acetone.
The process parameters recommended at the first stage of treatment of the resultant cellulose acetate and concerned with water concentration, the catalyst used, the temperature and duration of the process have been found experimentally and are the optimum ones under conditions of a low module.
The second stage is aimed at further reduction of the bound sulfuric acid content. This is attained due to a higher water concentration in the reaction mixture, thus making it possible to shift the reversible reaction of saponification of the sulfo groups in a required direction.
Using the parameters that we have found experimentally and recommend for the second stage, it is possible to decrease the bound sulfuric acid content of the finished product to hundredth fractions of percent. As distinct from the first stage of treatment, some saponification of the acetate groups occurs in the second-stage treatment.
Practical application of the proposed catalytic system of the aforestated composition, low module and corresponding temperature profile of the process in acetylation enables one to attain an optimum ratio between acetylation rates and cellulose destruction and thus to obtain the degree of polymerization of the finished product within 250 and 500.
If it is assumed that the rate of cellulose destruction in the course of its acetylation depends on the concentration of protons, it becomes easily understood why a possibility arises for controlling the degree of polymerization of the finished product in the presence of the proposed catalytic system. At the same time, the proposed catalytic system makes it possible to carry out synthesis of the soluble primary cellulose acetates. Presented below is a diagrammatic representation of the cellulose acetylation mechanism we have suggested. The principal part played by the adduct consists is bonding sulfuric acid into a strong complex that diminishes considerably the concentration of free sulfuric acid and its protons. As regards the cellulose sulfation process, it may proceed with participation of said complex or of free sulfuric acid. N,N-dimethylacetamide, in its turn, is an acceptor and carrier of protons with which it interchanges with acetic anhydride and acetic acid.
After desulfation the resultant acetate is completely soluble in acetone and is featured by a reduced viscosity of concentrated solutions thereof (that is, 1.5-2 times lower than that of diacetate produced according to the known so-called homogeneous process) and by a high filterability of acetone solutions which gives evidence of a chemical and physical homogeneity of the polymer being synthesized.
The aforementioned limits of the synthesis conditions provide for production of the acetone-soluble finished product, reduction of the acetylation module, sulfuric acid consumption and that of N,N-dimethylacetamide, as well as reduction of the amount of acetic acid forwarded for recovery.
One more field for the application of the catalytic system of the invention is for producing mixed cellulose esters, such as acetopropionates and acetobutyrates with low module. The process of the invention comprises activation of cellulose with acetic acid, preparation of a catalytic system and the acylating mixture, acylation of activated cellulose with the acylating mixture comprising at least the catalytic system and propionic anhydride or butyric anhydride, followed by hydrolysis and isolation of the end product. Wherein according to the invention, preparation of catalytic system is conducted in the absence of cellulose, and an adduct of sulfuric acid with N,N-dimethylacetamide, an adduct of perchloric acid with N,N-diniethylacetamide in the amount of 0.5 mole and free N,N-dimethylacetamide in the amount of max. 0.1 mole per 1.0 mole of an adduct of sulfuric acid with N,N-dimethylacetamide are used as the catalytic system. This system is added to the acylating mixture in the amount of 10.0-20.0 wt. % of sulfuric acid and 5.0-10.0 wt. % of perchloric acid in terms of said acids. Acylation is performed at a temperature of 10xc2x0 C.-90xc2x0 C., and module of about 5.0.
The proposed catalytic system offers unique possibilities for realization of an industrial process for the synthesis of cellulose esters with the lowest acylation module equal to 3-7 in a medium extensibly applicable in industrial practice, consisting of a lower fatty acid anhydride and a lower fatty acid, virtually without the use of any organic solvents.
The cellulose acetylation process and the process for producing acetone-soluble cellulose acetates according to the invention make it possible to synthesize:
(a) soluble primary cellulose acetates formed according to the xe2x80x9chomogeneousxe2x80x9d mechanism;
(b) cellulose acetates having an increased degree of polymerization equal to 300-500;
(c) cellulose acetates featured by high filterability and reduced viscosity of the concentrated solutions thereof;
(d) mixed cellulose esters, i.e., cellulose acetobutyrates and acetopropionates.
The processes mentioned above involve no use of a deficient raw material, are ecologically pure and economically profitable.